WASHINGTON — A pipeline that would greatly expand imports of oil sands crude from Canada won't significantly threaten water in the Great Plains or have much impact on climate change, the State Department argued in a final environmental impact statement it made public Friday.

While not the final go-ahead, the environmental assessment offered a preview of the Obama administration's pro-pipeline arguments in the face of efforts by environmental groups to get the United States to take action to reduce carbon emissions. The tarlike form of oil in the sands requires more energy to extract and process, and therefore produces more greenhouse gas emissions than conventional oil.

At the heart of the Obama administration's arguments is the contention that Canada will expand strip mining and drilling for the very thick oil in Alberta whether or not the Keystone XL pipeline to the Gulf Coast gets built.

"Climate change has gone too far. Keep the tar sands where they are," demonstrators chanted Friday as 54 people — some young, but many with gray hair — were arrested one by one in a peaceful protest outside the White House. So far 376 people have been arrested in seven days of protests.

After a series of upcoming public hearings, the Obama administration will make a final decision about whether the pipeline is in the national interest.

The State Department's environmental assessment argued that the pipeline is needed because refineries in Texas — which account for half the nation's refining capacity — have declining supplies of heavy crude from Mexico and Venezuela. The refineries could handle more heavy crude if the pipeline brings it from Canada, the report said.

It noted that a Department of Energy lab found that the greenhouse gas emissions of Canadian oil sands crude were 17 percent higher than gasoline from the average mix of crude oil consumed in the United States. But it went on to argue that the pipeline wouldn't have an effect on the amount of crude produced from the oil sands because Canada would find other ways to get it to market.

"The sense we have is that this oil sands is going to be developed and therefore there's not going to be any dramatic change in greenhouse gas if the pipeline was to go forward or without the pipeline," Kerri-Ann Jones, the assistant secretary of state for oceans and international environmental and scientific affairs, said in a briefing.

A proposed Canadian pipeline to a port on the Pacific Ocean has been hotly protested in Canada and so far is blocked.

The executive summary of the report said it was uncertain whether emissions from the oil sands would increase.

However, the companies in the oil sands increasingly are going after oil deep in the ground instead of scraping it off the surface. The drilling requires more energy and creates more emissions than strip mining. The Canadian environment ministry recently estimated that greenhouse gas emissions from the oil sands will triple from 2005 to 2020.

"The U.S. State Department's final report on the Keystone XL today is an insult to anyone who expects government to work for the interests of the American people," Sierra Club executive director Michael Brune said in a statement.

The American Petroleum Institute, a lobby group for the oil and gas industry, issued a statement saying it welcomed the government's report and wanted it to approve the permit quickly.

The proposed pipeline would run from the oil sands of Alberta through Montana, South Dakota, Nebraska, Oklahoma and Texas. An existing Keystone pipeline brings 591,000 barrels of crude from the oil sands to refineries in Oklahoma and Illinois.

The new pipeline would increase capacity to 1.3 million barrels a day and get some of the oil to refineries in Texas.

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The pipeline would cross the Ogallala Aquifer, the source of water for 30 percent of U.S. agricultural irrigation. A section also would cross Nebraska's Sand Hills, where the aquifer is at or near the surface.

The State Department report said that studies showed that in no case would a spill leave the entire aquifer "adversely affected." TransCanada, the company that wants to build the pipeline, would check for several years to make sure that reclamation was properly carried out in the Sand Hills, it said.

The State Department's analysts also considered other routes that would avoid the aquifer and Sand Hills but ruled them out after considering environmental conditions and costs.